The present invention relates to a process for forming trifluoromethyl-substituted compounds employing a low energy plasma as an energy source and to the trifluoromethyl-substituted compounds produced thereby.
Prior to the present invention, trifluoromethyl radicals have been formed by passing a trifluoromethyl-substituted organic compound as feed through a plasma formed by radio frequency discharges, which radicals are contacted with a substrate to form trifluoromethyl-substituted compounds. This low power glow discharge or corona discharge provides a convenient synthetic route to a large number of novel inorganic compounds by forming from the feed highly reactive radicals and atoms in excited electronic states at gas temperatures of from about 30.degree. to 100.degree.C. These reactive species are normally found only at high temperatures and are characteristic of temperatures in excess of 1000.degree.C. Thus, the "cold" plasmas, i.e. glow discharge or corona discharge, offers the advantage of high energy species at temperatures where chemical reactions can occur without degradation of the substrate or products.
The generation of trifluoromethyl radicals is a particularly interesting case. For example, the carbon-carbon bond in hexafluoroethane is anomalously weak due to the strong electron withdrawing effect of the six fluorine atoms. The best estimate of the actual bond strength at present is 68-82 Kcal/mole as compared to 86-88 Kcal/mole for the average carbon-carbon bond. The carbon-fluorine bond energies in hexafluoroethane are 114-119 Kcal/mole and thus one has a difference in bond energy of 32-51 Kcal/mole between the two types of bonds in the compound. When hexafluoroethane is introduced into the glow discharge, the initially available electrons in the gas gain energy from the applied radio frequency field. When the discharge is initiated, more electrons become available through partial ionization of radicals. The electron energies approximate a Maxwellian distribution and it is only the electrons in the high energy end of this distribution which are responsible for fragmentation of the hexafluoroethane upon collision. There are several available mechanisms by which fragmentation may occur. The two processes of lowest energy are the breaking of carbon-carbon and carbon-fluorine bonds, and these appear to be the most prominent processes in the low energy glow discharge region.
The most commonly employed source of trifluoromethyl radicals is the compound, hexafluoroethane, although other sources have been employed. At one time, it was thought necessary to trap the trifluoromethyl radicals in an inert solidified matrix at low temperatures prior to reaction with a substrate as exemplified by U.S. Pat. Nos. 3,196,115 and 3,240,691. Unfortunately, this process has substantial disadvantages which have seriously limited its use. While the frozen free radicals can be evolved subsequently from the solid matrix by heating the matrix, the radicals so evolved have very little energy so that they are reactive only with very limited substrates such as fluorine, chlorine, or bromine. Even with these substrates, the radicals are more reactive with themselves than with the substrate.
Alternatively it has been proposed to pass a trifluoromethyl radical source and the substrate simultaneously through an electrically induced plasma as shown in German DOS No. 2,060,351. As shown in the German publication, the substrate constitutes iodine which is decomposed in the plasma to form iodo radicals which combine with the trifluoromethyl radicals formed from hexafluoroethane. Unfortunately, such a process is not suitable if it causes the substrate to decompose into radicals which themselves may be decomposed further or which will react preferentially with each other, rather than with the trifluoromethyl radicals. For example, if a metal halide substrate were passed through the entire plasma admixed with the trifluoromethyl radicals, a substantial portion of the metal would plate out on the inner reactor wall thereby substantially reducing the yield of a trifluoromethyl-substituted metal compound. Also, when organic substrates are employed as the reactant, the organo radical formed in the plasma will itself become decomposed to form, among other by-products, coke. This, of course, is undesirable since the product yield will be reduced substantially.